Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Read on for more information about this type of cancer, the most common brain tumor to strike adults:

What is a glioma?A brain tumor is a mass of abnormal cells growing in the brain. The American Cancer Society estimates that 21,000 Americans a year are diagnosed with brain tumors; about 10,000 are gliobastomas. A glioma is any tumor formed in the glial, or supportive, brain cells, which are different from the better-known neurons.

Glioma is not a specific type of cancer. It is a general category of brain tumor that includes astrocytomas, oligodendrogliomas, ependymomas and gliobastoma multiforme. They are named according to the specific type of brain cell affected. Gliomas can be high-grade, which are more dangerous, or low-grade, which are often slow-growing and require little or no treatment.

Doctors still have not defined the kind of glioma that affected Kennedy. A glioblastoma is the most common brain tumor that affects adults. It is also the most rapidly growing malignant tumor of the brain, with the shortest survival rate. Death may occur within months.

“It’s the most malignant, the most aggressive, the tumor of the brain associated with the shortest survival,” said Marc Chamberlain, a professor of neurology and director of the Brain Tumor Program at University of Washington, Fred Hutchinson Cancer Research Center and the Seattle Cancer Care Alliance.

Cancerous tumors of the brain and spinal cord differ from tumors in other parts of the body, Otis W. Brawley, M.D., American Cancer Society's chief medical officer said in a statement. "Unlike tumors that start in the rest of the body, whose most deadly aspect is their ability to spread throughout the body, brain tumors almost never spread to other organs," Brawley said. "The most dangerous aspect of these tumors is that they can interfere with essential, normal functions of the brain."

The incidence rate for a glioma goes up with age and peaks in the age group from 75 to 84. Although a malignant glioma can occur in people with a history of radiation therapy for childhood cancer, or in those who have certain genetic conditions, that’s not likely the case in Kennedy’s situation, said Chamberlain.

“We don’t know what causes them,” he said. “For all practical purposes, they tend to come out of nowhere.”

What are the symptoms?
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Common symptoms of a brain tumor can include headache. Headaches linked to a brain tumor are often more painful in the morning and tend to improve throughout the day. The headaches, which can be accompanied by vomiting in the morning, may get worse with coughing or exercise or a change in position, such as kneeling.

About one-third of patients diagnosed with a brain tumor do not realize they are ill until they have a seizure.

How is a brain tumor diagnosed?
Brain tumors can be diagnosed by a basic neurological exam, including tests for eye movement, vision, hearing and reflexes. Brain scans, either an MRI or CT, can detect diseased brain tissue. Patients may also receive a surgical biopsy, where a small amount of the tumor tissue is removed for testing.

How is it treated? About 95 percent of patients will opt for surgery to remove as much of the tumor as possible, although because the tumor has usually spread deep into the brain by the time of diagnosis, it is rare for it to be completely removed.

Standard treatment involves some oral chemotherapy drugs which can reach into the brain to attack tumor cells. Radiation therapy can help prolong life. The Food and Drug Administration has also approved surgical implantation of chemotherapy wafers directly into the tumor. Radiosurgery is also performed.

Kennedy underwent targeted surgery to remove as much of the tumor as possible and a grueling regimen of radiation and chemotherapy.

Average survival can range from less than a year for very advanced and aggressive types — such as glioblastomas — or to about five years for different types that are slower growing. In typical glioma patients, ages 18 to 69, who have surgery and follow-up treatment, the average survival is 14.5 months, says Chamberlain.

Msnbc.com's JoNel Aleccia, the Brain Tumor Program at University of Washington, Fred Hutchinson Cancer Research Center and the Seattle Cancer Care Alliance, the American Brain Tumor Association, The Associated Press and Reuters contributed to this report